Liquid crystal display elements have become widely used, not only in watches and pocket calculators, but in all manner of measuring equipment, motor vehicle display panels, word processors, personal digital assistants, printers, computers and televisions and the like. Representative examples of liquid crystal display systems include TN (twisted nematic), STN (super twisted nematic), DS (dynamic scattering) and GH (guest-host) systems, as well as a FLC (ferroelectric liquid crystal) system which enables high-speed response. Furthermore, in terms of the drive system employed, conventional static drive and multiplex drive systems are the most typical, although simple matrix drive systems, and more recently active matrix drive systems, are now also being used in practical applications.
The liquid crystal compositions used in these types of systems are typically prepared by mixing two or more compounds, wherein the mixing ratio is determined so as to achieve physical properties (such as the nematic phase temperature range, the birefringent index (Δn), the dielectric anisotropy (Δ∈), the viscosity, and the elastic constant) and electro-optical properties (such as the response time, the threshold voltage, and the steepness of the V-T curve) for the liquid crystal composition that are appropriate for the targeted liquid crystal element display system and drive system. In most cases, the composition must have a high level of reliability when exposed to heat, light or moisture or the like. Further, in the case of an active matrix drive system, it is important that the liquid crystal composition also exhibits a satisfactorily high voltage holding ratio (VHR). In order to enhance the reliability, resistivity and voltage holding ratio (VHR) of a liquid crystal composition, a high level of reliability and a high voltage holding ratio (VHR) must be achieved for each of the liquid crystal compounds that constitute the composition. However, even if compounds having high levels of reliability, resistivity and voltage holding ratio (VHR) are used, the quality may deteriorate during production of the mixture of the compounds that functions as the liquid crystal composition.
Examples of the method used for producing the liquid crystal composition include methods that employ melt mixing under heat, and methods in which the liquid crystal compounds are dissolved in an organic solvent to effect mixing, and the organic solvent is then removed (see Patent Document 1). However, with these proposed methods, the quality of the composition often deteriorates. For example, in melt mixing method under heat, oxygen may cause oxidative decomposition of the liquid crystal compounds during the heating, resulting in a marked reduction in the resistivity and/or voltage holding ratio (VHR) of the liquid crystal composition. Moreover, the upper limit temperature of a liquid crystalline phase tends to fall, and the physical properties and electro-optical properties of the composition may change. In an organic solvent dissolution method, impurities or dopants within the organic solvent may cause a marked reduction in the resistivity and/or voltage holding ratio (VHR) of the liquid crystal composition. Moreover, residual solvent resulting from incomplete removal of the organic solvent also tends to cause a decrease in the resistivity of the liquid crystal composition and a marked reduction in the voltage holding ratio. On the other hand, production methods in which heating is conducted at a comparatively low temperature in a state of reduced pressure have also been proposed, and the production of liquid crystal compositions having a low resistance value have been disclosed (see Patent Document 2). However, although the method disclosed in the cited document enables production to be conducted at a comparatively low temperature, heating is still required, meaning the adverse effects associated with heating cannot be eliminated entirely, and heating equipment is still necessary. Moreover, another problem arises in that the method disclosed in the cited document also requires a large apparatus associated with the pressure reduction. In other words, in order to enable the liquid crystal compounds to be melted under reduced pressure, a container capable of withstanding the reduced pressure conditions is necessary, meaning the production apparatus inevitably becomes a large apparatus capable of withstanding reduced pressure. Further, pressure reduction is usually achieved using a vacuum pump, but this means that a trap must be provided to prevent backflow of the oil mist used in the vacuum pump, and a cooling device must also be provided to cool the trap. In this manner, production of a liquid crystal composition under reduced pressure tends to result in a significant enlargement of the production equipment, which would necessitate enormous capital investment to keep up with the increasing demand for liquid crystal compositions.
On the other hand, in recent years, high-speed response has become a much demanded property for liquid crystal compositions. In order to achieve such a high-speed response, a liquid crystal compound having a comparatively small molecular weight must be added. If preparation of the liquid crystal composition is conducted under reduced pressure, then a liquid crystal compound having a comparatively small molecular weight tends to volatilize, causing a change in the mixing ratio of the composition. Accordingly, the method disclosed in the above cited document is unsuitable for the production of liquid crystal compositions containing a liquid crystal compound having a comparatively small molecular weight.
Examples of known devices for agitating liquids include planetary mixing devices (see Patent Document 3), vibrators, lab mixers, stirring propellers, shakers and rotary evaporators. However, these agitating devices are typically designed as being used for the agitation of liquid crystal compounds in a liquid phase, and not designed as being used for the agitation of liquid crystal compounds in a powder form.
As described above, the development of a method for efficiently producing a high-quality liquid crystal composition using simplified equipment has been keenly sought.
Patent Document 1: Japanese Unexamined Patent Application, First Publication No. Hei 5-105876 (page 5, right column)
Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2002-194356 (page 4, Examples)
Patent Document 3: Japanese Unexamined Patent Application, First Publication No. Hei 6-71110 (Claim 3)